Screw Caused Columbia's Short Circuit

Nasa Says Power Tool Likely Stripped It

CAPE CANAVERAL - A stripped screw is to blame for the potentially serious short circuit that marred the July 23 launch of the space shuttle Columbia.

Seconds after liftoff, the short knocked out a pair of computers controlling two of the shuttle's three main engines. Backup computers took over and kept Columbia soaring toward orbit - but had another glitch shut down one of the backups, the shuttle would have lost an engine and might have been forced to make an emergency landing in Florida or Africa.

Engineers at Kennedy Space Center gained access to Columbia's midsection last weekend and traced the short to a wire running between one of the failed computers and the orbiter's flight deck. Beneath the payload bay, part of the wire and a nearby screw were blackened with soot.

``They followed the wiring back and found a place where the wiring makes contact with a Phillips head screw,'' said Joel Wells, a Kennedy Space Center spokesman. ``That contact caused the short.''

Mission managers theorize that, when the suspect screw was installed, a power tool slightly stripped the screw's head, leaving a small metal spur. During 25 previous launches and landings, vibrations caused the wire to rub against the spur, wearing away some of the wire's protective insulation.

When bare wire came in contact with the screw during Columbia's last launch, the resulting arc of electricity shorted out the entire circuit, tripping a circuit breaker and shutting down the two computers.

As a result, NASA decided Monday to take extra precautions for the shuttle Endeavour's planned Sept. 16 launch. They delayed until Wednesday Endeavour's scheduled ``rollover'' from its hangar to KSC's Vehicle Assembly Building for mating to the external tank and twin, solid-fuel rocket boosters.

Technicians will use the extra time to examine Columbia and the shuttle Atlantis for similarly damaged screws. Endeavour's payload bay already has been loaded, so a detailed check of the orbiter would be costly and possibly delay the launch. Instead, engineers will examine the other shuttles to make sure the problem was a fluke.

``We think we can get a good feeling about how Endeavour looks by looking at the others,'' said Bruce Buckingham, a KSC public affairs officer.

NASA also is wrapping up its investigation into a hydrogen leak in one of Columbia's engine nozzles that occurred during the same launch. The leak, which left the shuttle in an orbit 7 miles lower than planned, was caused by a nail-sized plug that popped out seconds before lift- off and struck the nozzle's hydrogen-filled coolant lines.